Ascorbic acid inhibits [3H]SCH-23390 binding to striatal dopamine D1 receptors

A common molecular motif characterizes extracellular allosteric enhancers of GPCR aminergic receptors and suggests enhancer mechanism of action

Several classes of compounds that have no intrinsic activity on aminergic systems nonetheless enhance the potency of aminergic receptor ligands three-fold or more while significantly increasing their duration of activity, preventing tachyphylaxis and reversing fade. Enhancer compounds include ascorbic acid, ethylenediaminetetraacetic acid, cortico-steroids, opioid peptides, opiates and opiate antagonists. This paper provides the first review of aminergic enhancement, demonstrating that all enhancers have a common, inobvious molecular motif and work through a common mechanism that is manifested by three common characteristics. First, aminergic enhancers bind directly to the amines they enhance, suggesting that the common structural motif is reflected in common binding targets. Second, one common target is the first extracellular loop of aminergic receptors. Third, at least some enhancers are antiphosphodiesterases. These observations suggest that aminergic enhancers act on the extracellular surface of aminergic receptors to keep the receptor in its high affinity state, trapping the ligand inside the receptor. Enhancer binding produces allosteric modifications of the receptor structure that interfere with phosphorylation of the receptor, thereby inhibiting down-regulation of the receptor. The mechanism explains how enhancers potentiate aminergic activity and increase duration of activity and makes testable predictions about additional compounds that should act as aminergic enhancers.

Antagonistic Activity of Ascorbic Acid (Vitamin C) on Dopaminergic Modulation: Apomorphine-Induced Stereotypic Behavior in Mice

Among the various neurotransmitter systems implicated in the mechanism of action of ascorbic acid (vitamin C), the relationship between the dopaminergic system and ascorbic acid is not particularly clear. Ascorbic acid is speculated to have an antagonistic effect on dopaminergic modulation. With this background in mind, in the present study we have seen the effect of ascorbic acid per se and in combination with typical and atypical antipsychotic agents against apomorphine-induced stereotypic behavior in mice. Male Laka mice weighing 20-25 g were used in the present study. Apomorphine-induced stereotypic behavior was used as an animal model. Various dopaminergic modulators were used. Ascorbic acid dose-dependently inhibited stereotypic behavior produced by apomorphine in mice. It potentiated the antipsychotic activity of haloperidol (0.1 mg/kg i.p.), a typical antipsychotic agent. When administered along with atypical antipsychotics, clozapine (1-2 mg/kg i.p.), sulpiride (10-20 mg/kg i.p.) and risperidone (0.0025 mg/kg i.p.), ascorbic acid also potentiated their activity. Also when given along with SCH-23390, a selective D(1) antagonist, an additive effect was observed. Ascorbic acid also inhibited the supersensitization response of apomorphine on reserpinization (2 mg/kg i.p.). Interestingly, at a lower dose (100 mg/kg i.p.), ascorbic acid potentiated the dopaminergic activity of apomorphine (0.5 mg/kg) and BHT-920 (0.25 mg/kg i.p.). However, when given concomitantly with SKF-38393, it failed to alter the response of SKF-38393. The data substantiate the hypothesis that ascorbic acid potentiated the activity of typical as well as atypical antipsychotics and that the effect of ascorbic acid on the dopaminergic system is markedly dose dependent; a low dose (100 mg/kg i.p.) potentiated the dopaminergic action while higher doses (400-1,600 mg/kg i.p.) blocked it.

Ascorbic acid antagonizes ethanol-induced locomotor activity in the open-field

It has been reported that ascorbic acid (AA) antagonizes the physiological and behavioral effects of dopamine (DA). AA reduces locomotor activity induced by dopaminergic agonist drugs. Also, AA amplifies the action of antidopaminergic drugs. Ethanol, like other drugs, produces a release of DA in the mesolimbic pathway, and at some doses, induces locomotor activity in mice. The ethanol-induced locomotor activity could be dopamine-dependent because it can be reduced by antidopaminergic drugs. In the present study, we investigated whether an acute administration of AA reduces ethanol-induced locomotor behavior. AA, at doses (0.0, 21.85, 87.5, 175, 350. and 1400 mg/kg) was injected i.p. into mice, 0, 30, 60, or 90 min before an i.p. injection of ethanol (0.0, 0.8, 1.6, 2.4, and 3.2 g/kg). Locomotor activity was evaluated in open-field chambers. Our results showed that AA (350 and 1400 mg/kg) reduced ethanol-induced locomotor activity when injected 30 min before ethanol treatment. This effect was lost when ethanol was administered 90 min after AA injection. AA also reduced locomotor activity produced by d-amphetamine and methanol. The results support a pro-dopaminergic action of ethanol, and suggest a common dopaminergic pathway for the drugs of abuse in locomotor activity.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding sites for 5-hydroxytryptamine-2 receptor agonists are predominantly located in striosomes in the human basal ganglia

Previous autoradiographic studies have shown that serotonin 5-HT2 receptors are homogeneously distributed in the human striatum. While these studies were done using antagonist radioligands such as [3H]ketanserin, we describe here a heterogeneous distribution of 5-HT2 binding sites in the human striatum, using [3H]LSD and [125I]DOI as ligands. Beside their agonist properties, these compounds belong to the family of psychedelic drugs. The localization of their binding sites in the dorsal striatum is very similar to that of striosomes, as visualized by acetylcholinesterase histochemistry or [3H]flunitrazepam labelling. This heterogeneous distribution seems to be a peculiarity of the human and guinea-pig brain, for it is not found in the monkey, cat, pig, and cow. In the rat striatum, a weak patchniness was seen, but corresponded to 5-HT1C binding sites. The density of [125I]DOI binding sites over striosomes presents large variations, which can neither be correlated with parameters such as age, gender and post-mortem delay nor with the effects of neurodegenerative disorders, with the exception of Huntington's disease, at late stages of the disease. The drug binding profile of [125I]DOI binding sites in the striosomes is identical to that of matrix binding sites. It is also fully comparable to the pharmacological profile of cortical 5-HT2 sites reported using [3H]ketanserin as a radioligand, with the exception of the higher affinity displayed by agonists for [125I]DOI binding sites. Interestingly, biphasic displacement curves yield a better fit for spiperone, cinanserin and ketanserin competitions. This biphasic profile can probably neither be accounted for by the presence of 5-HT1C sites nor by the existence of multiple affinity states. Taken together, these data suggest that a heterogeneous population of 5-HT2 receptors is present on the cell bodies or dendrites of striosomal neurons. These receptors provide an additional anatomical substrate to explain the psychedelic action of indoleamine (LSD) and phenylethylamine (DOI, DOM) drugs.

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.